How do I fill the space in the centre of my circuit in 2.79?
If I understand correctly, what you could do:


First clean your mesh a bit, make W > Remove Doubles to remove the doubles and merge vertices, and untriangulate with AltJ.





Now as you have merged the vertices you can select the rim with a simple Alt left click, fill the face with F and join some vertices so that its more blender friendly. It is still a ngon though, do you need a quad grid?